The present invention relates to an improved corona device for modifying the surfaces of materials generally and of thermoplastic films in particular.
In processing films as well as in manufacturing composite films, it is known in the prior art to subject the film surfaces to a corona treatment in order to render them suitable for printing or to increase their bond strengths.
This method involves passing the film to be treated over an electrically grounded support surface, such as a roll, a drum, or an endless belt, and subjecting the film surface which is not in contact with the support surface to a corona produced by supplying high-frequency, high-voltage A. C. to an electrode arranged at a distance from the support surface.
The known methods and devices operating according to this basic principle differ in reality only in the design of the support surface serving as a counter electrode. These surfaces may, for example, comprise a central roll with several electrodes, several support rolls with corresponding electrodes or the like. The dielectric materials used to insulate the counter electrode are, for example: mica, glass, ceramics, plastic films or special qualities of rubber. The electrodes conventionally employed may comprise a plate, wire, comb, knife, half-shell, spring or spindle-shaped electrode. The type of generator used may, for example, be a low-frequency, medium-frequency or high-frequency generator. In addition, methods are known such as those described in German Offenlegungsschrift No. 1,4 04,413, U.S. Pat. No. 2,864,755 and U.S. Pat. No. 2,802,085 in which the above-described conditions have been reversed, i.e., the film is subjected to an electrical corona discharge through an electrode insulated by means of a dielectric material.
The basic systems just described (bare electrode/insulated counter electrode or insultated electrode/bare counter electrode) show general imperfections which are more or less troublesome in practice.
When using a bare electrode and an insulated counter electrode, one drawback, among others, is the relatively high cost resulting from damage to the insulation of the counter-electrode due to punctures, injuries such as cuts or the like or the introduction of moisture into the pre-treating station. In such cases, the rolls which are usually insulated by special rubber layers, silicone, etc. must be sent to a rubberizing and vulcanizing plant for repair. As is known from experience, the repair procedure is time-consuming and costly, and the vulcanizing plant is obliged to keep expensive spare parts in stock.
The problems outlined have induced the development of the alternative method in which preferably cylinders or rolls covered with a dielectric material are used as electrodes, as already mentioned. The use of small electrode rolls certainly has some advantages. Apart from a simple design and easier handling when mounting and dismounting, there is also a reduction in the cost of repair achieved by using exchangeable dielectric linings in the form of tube materials which may be fitted or shrunk on, instead of the vulcanizable, permanent insulating layers. As far as the effectiveness of the pre-treatment is concerned, i.e., the surface tension in mN/m obtained on the treated substrate, the first-mentioned method is definitely superior to the alternative method, due to the possibility of combining the counter electrode, necessarily designed as a continuous surface (insulated cylinder), with a point electrode of any shape (wire, comb, threaded pin or knife-shaped electrode, etc.). This is all the more understandable when one considers the preferred electrode shape, namely, a cylindrical roll body of 80 to 100 mm diameter which is thus very far from the sharp-edged electrode profile which is generally regarded as ideal.
German Utility Model No. 74 14 967 would appear to be an attempt to solve the problems mentioned above. It suggests using profiled bare electrode rolls in combination with an insulated counter electrode roll. There are, however, no apparent advantages as compared to a stationary electrode profile (knife, comb-shaped electrode, etc. and, additionally, the desirable principle of the insulated electrode has been abandoned. Consequently, the patented device can only be regarded as a compromise resulting from the necessity of improving a system which is not optimal and the desire to maintain a given concept.
In addition, German Pat. No. 2,044,828 specifies designs for a corona device comprising a roll-like body composed of rods and having electrode wires arranged inside for removing a charge from the underface of a film passed over them, while the upper surface of the film is discharged by electrode wires disposed above the roll body. However, this corona device merely serves to remove charges from films, and it is not adapted for modifying the surfaces with a view toward an improved suitability for printing and/or superior bond strengths.